1. Field of the Invention
The present invention relates to an information receiving system, in particular to a broadcast receiving system, capable of receiving two kinds of information or broadcast radio waves. The broadcast receiving system may receive an FM radio broadcast and a FM multiplex broadcast for superimposedly displaying traffic information on a monitor displaying a road map. Furthermore the present invention relates to an information receiving method, in particular to a broadcast receiving method, using the inventive information receiving system.
2. Description of the Prior Art
In a system wherein an FM multiplex receiver and an FM radio are constructed by two tuners, basic function elements of the FM multiplex receiver 1 and the FM radio 2 are provided independently of each other as shown in FIG. 6. Accordingly, there is no correlation between a tuning frequency (multiplex receiving frequency) f0 of the FM multiplex receiver 1 and a tuning frequency (radio frequency) fl of the FM radio 2.
Generally, in a superheterodyne receiver, though to a very small extent, local oscillation caused when a specific receiving frequency is selected is leaked as noise radio waves from an antenna or the like.
For example, when fIF in FIG. 6 denotes an intermediate frequency of the FM multiplex receiver 1 and the relationship between the multiplex receiving frequency f0 and the radio frequency f1 is defined as f0-fIF =f1, if an electric field intensity from an FM radio station is weak, it is added onto a local oscillation frequency (f0-fIF) leading from the antenna of the FM multiplex receiver 1, with the result that noise is included in the sounds from the FM radio 2. In other words, the condition f0.noteq.f1+fIF should be met, in order to avoid the generation of noise.
In order to mitigate the above problem, in a prior art system, an isolation buffer 3 is provided between the FM multiplex receiver 1 and the FM multiplex antenna, as shown in FIG. 7, to attenuate the local oscillation leaking from the FM multiplex receiver 1, thereby reducing an influence on the FM radio 2.
By attenuating the noise acting in a direction opposite to a normal direction by providing the isolation buffer 3 as described above, if the level of the local oscillation leaking from the FM multiplex receiver 1 is, for example, -60 dBm, the level of the noise leaking from the antenna is suppressed to -100 dBm or lower if the isolation level of the isolation buffer 3 is 40 dBm. As a result, the leakage of the local oscillation gives almost no influence on the FM radio 2.
However, the external provision of the isolation buffer 3 complicates the system. Even if the isolation buffer 3 is provided in the FM multiplex receiver 1, it similarly leads to a larger receiving system.
An object of the present invention is to provide an information receiving system capable of eliminating the influence of noise due to the leakage of local oscillation without additionally providing an isolation buffer, and an information receiving method using such a system.